There is an increasing requirement to recycle materials such as aluminum magnesium and other metals and non-metals. Often such materials will be coated in paint, oil, water, lacquers, plastics, or other volatile organic compounds (V.O.C.s) which must be removed prior to remelting the materials. For materials which are capable of being processed at relatively high temperatures without melting, such impurities are typically removed using a thermal process which is sometimes known as de-coating. Such thermal de-coating processes can also be used to dry and/or sterilize materials prior to remelting.
For example, aluminium is often used in the production of beverage cans which are typically coated in paint, lacquers and/or other V.O.C.s. Before used beverage cans (U.B.C.s) or scrap material produced during the manufacture of beverage cans can be melted down for recycling, any coatings or other impurities must be removed in order to minimize metal loss.
Thermal de-coating, however, is not limited to application to aluminium but can be used to clean or purify any metal or non-metallic materials which are capable of withstanding the temperatures present in the thermal de-coating process. Thermal de-coating can be used to de-coat or purify magnesium or magnesium alloys for example.
Known thermal de-coating processes involve exposing the material to be treated hot gases in order to oxidise the coatings and/or impurities which are to be removed. This exposure takes place in a closed environment in which the temperature and oxygen content of the hot gases can be controlled. Temperatures in excess of 300 C. are required to remove most organic compounds and an oxygen level in the range of 6% to 10% is normally required.
If the temperature and oxygen levels of the hot gases are not carefully controlled the process can go autothermic as the V.O.C.s which are released during the thermal stripping are combuted. This can result in an uncontrolled increase in the temperature of the hot gases which may be very dangerous.
The material will usually be shredded before treatment and it is important for effective de-coating that all the surfaces of the shredded material are exposed to the hot gases. If this does not occur then the treatment becomes less effective and, in the case of U.B.C.s in particular, a black stain may be left on the surface of the treated material. It is also desirable for the material to be agitated during the treatment to physically remove lose coatings or impurities from the material.
At present there are three main systems which are used for thermal de-coating, these are:
1. Static Oven
In static oven, the material is stacked on a wire mesh and hot gases are recirculated through the oven to heat the material to the required process temperature.
This arrangement is not efficient because the hot gases do not come in to contact with the materials that are enclosed within the stack of materials on the mesh. As discussed previously, it is important in de-coating that all the surfaces of the materials being treated are exposed to the hot gases. Also there is no agitation of the material being treated.
2. Conveying Oven
This system uses a mesh belt conveyor to transport materials for treatment through an oven. Hot gasses are passed through the material on the belt as it passes through the oven. The problems with this method are as follows:                The depth of materials on the belt limits the process. The materials are stacked, causing similar problems to those found with the static oven in which materials at the centre of the stack do not come into contact with the hot gases        There is no agitation of the materials, so loose coatings are not removed.        The conveyor belt life is short.        The materials have to be constantly fed.        The process is not suitable for low volume or continuously changing product.3. Rotating Kiln        
A large kiln is inclined to the horizontal so that material fed or charged into the kiln at its highest end travels towards the lowest end, where it is discharged, under the influence of gravity. The kiln is rotated so that material within the kiln is agitated and a flow of hot gases is provided to heat up the material as it travels through the kiln. A number of problems are associated with this method:                The material has to be constantly fed.        The process is not suitable for low volume or continuously changing product.        The continuous process requires air locks at both ends, materials charge end and materials discharge end.        The kiln requires a rotating seal leading to a high level of maintenance.        